Fin Rot: shredded or frayed tail or fins / Mouth rot (cotton wool mouth disease)

  • Main symptoms: The fins (very often the caudal fin, “the tail”) look eroded/shredded. In the initial phase, they appear bordered by an opaque or milky white border at the tip. Sometimes: red veined fins, hemorrhages. The symptoms and infection are caused by a type of bacteria called Flexibacter Columnaris or Flavobacterium Columnare. Can also affect the fish mouth (mouth rot).
  • Secondary symptoms: Loss of color, lethargy, loss of appetite. The affected fish may isolate itself from the group. Secondary fungal infection is not uncommon.
  • Direct causes: Most often bacteria (Flavobacterium columnnare) are the cause. Sometimes it can be ectoparasites that cause the disease (Pseudomonas).
  • Contributing factors: Stress factors: poor water quality (check the water quality: it should have 0 ammonia, 0 nitrite and low nitrate.). Unsuitable diet (outdated food) or overfeeding. Overcrowded tank. Physical damage (from decoration, stone or nipping from another fish). Water changes with water that is too cold. Overwintering of Japanese goldfish outdoors in cold areas.
  • Severity: Columnariosis is a severe disease and can be contagious, especially if other fish nip the infected fins. It’s treatable but it is important to start the treatment as early as possible as  this disease can escalate rapidly. If the base of the caudal fin is affected, the fish will unfortunately most likely die. Secondary infections are also very common (e.g. fungi).
  • Treatments: Commercial products – API Melafix, King British Bacteria Control, Esha 2000, Interpet Disease Treatment, Sera Baktopur – or veterinary prescription (antibiotics). Continue treatment for the length of time recommended, as ending treatment too early can result in a re-occurrence of the infection.
Fish with fin rot - close picture of caudal fin

The caudal fin of this fish is ragged and milky

Exclamation markImportant: do not forget to remove the activated charcoal during the entire treatment (as it will absorb the medication). It can be put back in the aquarium at the end of the treatment to remove the residues.
Good hygiene and water quality can prevent the re-occurrence of this disease.
Exclamation markImportant: Whilst all care is taken to provide accurate information on this website, Web Goldfish accepts no responsibility in the case of mis diagnosis of diseases/ incorrect treatments or any other errors made. In case of doubt, please always consult a veterinarian.